Millennials are crucial to Clinton's coalition, but polls show that while they strongly dislike Trump, they're far more interested in third-party candidates than voters overall

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(L-R) Gary Johnson and Jill Stein. A conservation group closely aligned to Hillary Clinton will be sending out letters to voters with their argument that voting for a third party candidate will only strengthen Donald Trump.

A deep-pocketed environmental group aligned with Hillary Clinton will blanket 1.1 million households in battleground states with mailers warning millennials that a vote for a third-party candidate only helps Donald Trump, the group told NBC News.

The League of Conservation Voters plans to spend $2.6 million before Election Day, most of which will go towards their efforts to prevent Libertarian nominee Gary Johnson and Green Party nominee Jill Stein — polling at about 7 percent and 3 percent, respectively — from siphoning votes away from Clinton.

"There are high stakes for young voters in this election, including the opportunity to meet the climate crisis head-on, and they overwhelmingly dislike Trump. But some may still be leaning towards a third-party candidate instead of Hillary," said LCV National Campaigns Director Clay Schroers. "This is a group of young people who don't want to risk a Trump presidency, and it's important that they know that a vote for anyone but Hillary is a vote for Trump."